The positions assigned to these
stars, though obtained with wonderful skill, considering the many
difficulties under which Tycho laboured, were quite inaccurate when
judged by our modern standards. Tycho's instruments were necessarily
most rudely divided, and he had, of course, no telescopes to aid him.
Consequently it was merely by a process of sighting that he could
obtain the places of the stars. It must further be remembered that
Tycho had no clocks, and no micrometers. He had, indeed, but little
correct knowledge of the motions of the heavenly bodies to guide
him. To determine the longitudes of a few principal stars he
conceived the ingenious idea of measuring by day the position of
Venus with respect to the sun, an observation which the exceptional
brightness of this planet rendered possible without telescopic aid,
and then by night he observed the position of Venus with regard to
the stars.
It has been well remarked by Mr. Baily, in his introduction to the
"British Catalogue of Stars," that "Flamsteed's observations, by a
fortunate combination of circumstances, commenced a new and a
brilliant era. It happened that, at that period, the powerful mind
of Newton was directed to this subject; a friendly intercourse then
existed between these two distinguished characters; and thus the
first observations that could lay any claim to accuracy were at once
brought in aid of those deep researches in which our illustrious
geometer was then engaged.
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